You're right, I was being over-specific there and there's plenty of maps that work perfectly fine without it specifically. Still, Viaduct and Sawmill have obvious industrial objectives, and Harvest has a ghost, for instance. I focused on spytech specifically because it's a cheap way to show there's something valuable there. I do think it serves an important purpose in DeGroot Keep (as far as anything about DeGroot Keep is important) in saying "This takes place at the same time as the other maps; they're fighting with swords and not guns because of Reasons, probably very stupid ones."
And yeah, the map's fun enough that I'd want to keep playing it even with dev textures, so tone isn't a dealbreaker. Still, right now it's so somber that I have a hard time reconciling it with the rest of TF2, and I feel like getting the tone in the right place would take it from "excellent custom map" to "Valve quality." After all, this is a game where the characters are employed by a guy who forgot to pay them because he was too busy wrestling a yeti, so even if the setting itself isn't zany it should be a place where zany things happening wouldn't be out of place. Of course it's possible to take things too far, so even though it's a fun map a lot of people regard koth_lolcano as a gimmick rather than a real map, so it's clearly possible for improper tone to hurt a map.
So basically I think it should have something suggesting military or industrial value, and something that suggests that rocket jumping soldiers and air-guritaring ax murderers aren't as out of place as they currently feel in a Japanese shrine. Still, you're the one actually putting in the work to make the thing so if you still disagree I won't push it further. In any case I'll do what I can spread the word about Suijin, especially since it's hit a relatively stable state.